Study of Pembrolizumab with Chemotherapy and Hormone Therapy for Early-Stage ER+/HER2- Breast Cancer

Inclusion Criteria

• Have a confirmed diagnosis of localized invasive breast cancer of a certain size and stage.
• Have estrogen receptor-positive, HER2-negative, Grade 3 breast cancer.
• Provide a recent biopsy of the breast tumor for testing.
• Have a good performance status, meaning they are fully active or have some restrictions but can still do light work.
• Agree to use effective contraception during the study and for a specified time after the last treatment dose.
• Have adequate organ function as determined by medical tests.

Exclusion Criteria

• Have a history of lung inflammation requiring steroid treatment or current lung inflammation.
• Have breast cancer with lobular histology or cancer in both breasts.
• Have metastatic breast cancer or multiple tumors in different areas of the breast.
• Have certain advanced stages of lymph node involvement.
• Have breast cancer that is estrogen receptor-negative and progesterone receptor-positive.
• Have had certain types of breast surgery before starting the study treatment.
• Have another invasive cancer that is progressing or required treatment in the last 5 years, except for certain skin or cervical cancers.
• Have an immune deficiency or are on long-term steroid therapy.
• Have an active autoimmune disease requiring treatment in the past 2 years.
• Have a history of active tuberculosis or an active infection needing treatment.
• Have significant heart disease or a low heart function.
• Have a history of HIV, hepatitis B, or active hepatitis C infection.
• Have received prior treatment for breast cancer or certain immune therapies.
• Have received a live vaccine recently.
• Have severe allergies to study medications.
• Are pregnant, breastfeeding, or planning to conceive during the study and for a specified time after treatment.